There is global interest in the development of alternative energy sources especially wind power.
The electrical power generated by such wind machines is often transmitted considerable distances to centres of population, and one reason for this is that people usually live in sheltered terrain where the wind resource is more modest. The less concentrated energy in the wind in such areas reduces the economics of energy capture with conventional propeller wind turbines.
It has been proposed to develop devices that concentrate wind energy by increasing wind velocity, thus enabling wind with a lower velocity and energy density to be used where it is available. These proposals can potentially eliminate long power transmission lines.
One approach is a diffuser duct, which increases in cross-sectional area downstream of the propellor. This duct creates a lower pressure, thus drawing a faster air stream through the smaller diameter air inlet. Problems with diffusers typically include the control of turbulence when the airflow breaks away from the inner surface of the duct. This turbulence restricts flow and substantially reduces the benefit of a diffuser as wind velocity increases. U.S. Pat. No. 4,075,500 and U.S. Pat. No. 4,422,820 describe ducts which propose control of such turbulence by introducing external airflows through a series of orifices.
A similar arrangement is shown in U.S. Pat. No. 4,132,499, which includes a separate annular aerofoil. Other proposals are found in WO-A-00/50769 and WO-A-01/06122.
Another way of concentrating wind energy is described in U.S. Pat. No. 2,485,543 and U.S. Pat. No. 2,784,556. In this case a wind machine operates on the depression principle and has a wind driven propeller with hollow blades and apertures at the tips. Rotation of the propeller induces air in the hollow blades to flow out through the apertures in the blade tips. This in turn causes a lower pressure within the device, and by communicating through an air passage a faster stream of external air is drawn through the system.
An aim of this invention is to improve energy conversion of a conventional turbine by increasing the velocity of the local fluid stream.